1. Field of the Invention
The present invention relates to an acoustic insulation of a multi-walled plane box and more particularly of an insulating glazing.
2. Discussion of the Background
Insulating glazings, comprising two or more glass sheets assembled together by way of an insert frame which keeps them a certain distance apart while trapping a gas space between them, in general a dry air space, are used in most cases for improving thermal insulation of buildings or possibly even of land transport vehicles.
The most widespread systems use glasses with a common thickness of 4 mm separated by a gap of between 6 and 12 mm. As such, these glazings have limited acoustic performance, substantially below that of a monolithic glass of the same overall mass per unit area.
In industry, various means are used to improve the acoustic performance of insulating glazings. The most widespread comprise using glasses of great and different thicknesses. This means has limited efficacy, in that it raises the weight of the glazing and this may compel the use of a reinforced window, and moreover, the increase in cost is not negligible. Another widespread means comprises replacing the monolithic glasses by laminated glasses. Here, the efficacy is limited and the rise in cost is even greater than in the previous case.
A means which is difficult to use for glazings intended to equip windows but which is widespread in inside partitions or in railway vehicles comprises raising the thickness of the air space. However, the effect is noticeable only for air thicknesses of several centimeters (5 or 6), and this prevents production of such variants in sealed insulating glazings.
An efficacious means lies in the use of special laminations. Thus, the European Patent Application EP 100 701 B1 describes an insulating glazing including one or two laminated elements whose resin is "such as a bar 9 cm in length and 3 cm in width, consisting of a laminated glass comprising two glass sheets 4 mm in thickness, joined together by a 2 mm layer of this resin, has a critical frequency which differs at most by 35% from that of a glass bar having the same length, the same width and 4 mm in thickness". The working principle of this type of glazing based on a low stiffness of the resin, independently of its damping, allows a very marked improvement in comparison with the ordinary laminated glazing but the price thereof is also noticeably higher.
Certain studies have proposed the use of panels of standard thickness and the installation at the periphery of the box of Helmholtz resonators tuned to the cavity of the box (see MASON and FAHY, Journal of Sound and Vibration, Vol. 124 (2), pages367 to 379, Academic Press Ltd 1988). Patent Application WO-A-85 02640 thus proposes a box with an improved acoustic insulation at certain frequencies. In one of its variants it includes localized spherical resonators situated outside the box and in communication with the internal volume via ducts of small cross-section. This system, since it is tuned to one frequency, acts especially within a region around this frequency, but, in regard to the other frequencies, either it acts little, or else it acts negatively. Moreover, the volume of the resonator must be a sizeable fraction of that of the cavity proper--of the order of 15%--which, for example, for a 1 m.sup.2 glazing with a 12 mm thick air space necessitates the installation at the periphery of the glazing of several "cylinders" whose total capacity is close to 2 liters; this solution is not suited to the customary conditions for manufacturing insulating glazings nor generally to the acoustic insulation of boxes of comparable thickness.
A variant resonator is also known in which the resonator/inside of the box link is not made via pipes as in the general case of Helmoltz resonators, but via a continuous link. Patent application DE-A-34 01 996 thus proposes a glazing whose glasses have different thicknesses and are mechanically detached from one another. A cavity of very large cross-section has been made at the periphery of the glazing and this makes it possible, taking into account its volume and the characteristics of the continuous slot which links it to the inside of the glazing, to tune the resonator in order to improve the insulation at a given frequency.
A variant of the glazings is also known which includes an absorbent peripheral material in which the latter is contained in a tube situated inside the glazing, at its periphery. Patent Application DE-A-27 48 223 proposes that the link between the inside and the outside of the tube containing the absorbent material be made via a very long narrow channel which extends from one end of the tube to the other. The working principle of this type of glazing is to cause the damping of sounds by an appropriate absorbent material. No variant without absorbent material is envisaged in DE-A-27 48 223.